Open Ableton Live, go to Preferences. Click on the "Link MIDI" tab, and set the output of a virtual MIDI bus to Sync On. In the example below, the IAC Driver (Bus 1) virtual MIDI bus is used.

Open up the Max patch. Select the virtual MIDI bus from the menu.

Go to the options menu and select Audio Setup. Set the output device to the $2 USB audio device.

Connect the headphone output of the USB sound card to the external input
of the Pocket Operator. Set the Pocket Operator so SYN2 (Sync In /
Stereo Out). Set the Pocket Operator to play. Pressing play in Ableton Live should now synchronise both devices.